Pressure sensitive adhesives that are ultra violet (UV) curable have been successfully used for label and tape applications for several years. Often the pressure sensitive adhesives (PSA) are polyacrylate based PSAs. Polyacrylates possess a variety of advantages over other adhesives because they are highly stable toward UV light, oxygen, and ozone which are found in abundance in the environment. Synthetic and natural rubber adhesives normally contain double bonds, which make these adhesives unstable to the aforementioned environmental effects. Further advantages of polyacrylates include their transparency and their serviceability within a relatively wide temperature range.
Polyacrylate PSAs are generally prepared in solvent solution by free radical polymerization of acrylic monomers and are then applied to a substrate from a solution using a coating bar. After application solvent is removed and they are dried on the substrate. Cohesive strength of an adhesive refers to the bond strength between the molecules of the adhesive while adhesive strength refers to the bond strength between the adhesive and a substrate. To increase its cohesive strength, the polymer is often crosslinked to a limited degree. Crosslinking or curing is done by exposure to either a thermal source or a UV source. This solvent process described is fairly costly and, as a general rule, the solvent is not recycled resulting in the high consumption of organic solvents and a high environmental burden. Moreover, it is very difficult to produce PSA tapes with a high adhesive application rate without also producing bubbles in the adhesive layer. One remedy is to use a hot melt process to apply the adhesive to a substrate. In this process, the PSA is applied to the backing material from the melt.
Hot melt pressure sensitive adhesives (HMPSAs) are thermoplastic compositions that combine the processing advantage of hot melt adhesives and the properties of pressure sensitive adhesives. Hot melt adhesives are solids at room temperature, melt at elevated temperatures, and enable easy coating on a substrate. Hot melt adhesives do not contain water or any solvents. They regain their solid form on cooling to form a permanently tacky solid coating on the substrate that adheres on contact to another substrate. These compositions are commonly applied to various substrates, such as paper, fabric, metal, and plastic films that are then converted into a large number of different products, especially pressure sensitive adhesive tapes and labels. These pressure sensitive adhesive products have a broad field of application in the automobile industry for fastening or sealing, in the pharmaceutical industry for bandages or transdermal drug delivery systems, and in the packaging industry for sealing, bonding or labeling.
Very low molecular weight polymers will yield hot melt pressure sensitive adhesives with sufficient fluidity, however the resulting adhesives lack cohesive strength. Very high molecular weight polymers give better cohesive strength, however these are often too viscous at the common application temperatures of from 175° F. to 356° F. to be easily coatable onto substrates. Achieving useable hot melt pressure sensitive adhesives requires balancing these two competing issues. To avoid the undesirable viscosity problems, polymers of moderate molecular weight have been made with various functional groups, photoinitiators, which undergo controlled crosslinking reactions upon exposure to UV radiation. In this manner, the cohesion of acrylic PSAs can be raised by providing sufficient crosslinking; however there still remain issues with hot melt viscosity and stability during application. The instability exhibits itself as a dramatic rise in viscosity of the HMPSA in the hot melt as it is heated. The viscosity instability can arise from incompatible functional groups on the polymer backbone or from unstable linkages such as urethane bonds in the polymer. The rapid increase in viscosity over a short period of time defeats the ability to apply the HMPSA to the substrate and thus the pot life of the typical HMPSA is very low.
It is desirable to create photoinitiators that can be polymerized into the polymer backbone in the HMPSA and that will provide sufficient crosslinking ability while maintaining the hot melt viscosity stability of the HMPSA at the typical usable temperatures.